Implantable medical devices for providing electrical stimulation to body tissues, for monitoring physiologic conditions and for providing alternative treatments to drugs, are well-known in the art. Exemplary implantable medical devices include, for example, implantable cardio defibrillators, pacemakers, and programmable neurostimulator pulse generators. The medical devices typically incorporate a power source connected with an electronic circuit having a circuit board in a hermetically sealed housing. Connected to the sealed housing often a header assembly is provided which includes electrical contact elements that are electrically coupled with the electronic circuit and/or to the power source located inside the housing via a feedthrough component. The header assembly provides a connector for electrical communication via an external medical lead cable.
Wireless communication with such implantable medical devices has become increasingly more important. Communication is necessary, for example, to program such a device, to monitor its various functions and to provide data concerning a patient's response to the devices therapy. Therefore, radio frequency (RF) transmissions are commonly employed to communicate with an implantable medical device. Therefore, such device often provides an RF telemetry antenna for transmitting or receiving signals.
U.S. Pat. No. 7,317,946 shows and describes an implantable medical device with an elongated antenna within the header for far field telemetry to ensure telemetry over distances of a few to many meters from an implantable medical device. The antenna is disposed outside the hermetically sealed housing within the header and has a serpentine arrangement, as one factor to consider for far field telemetry is the length of the antenna. In contrast, the dimensions of a medical device have to be as small as possible. However, the disclosed serpentine antenna provides a resonant length which is only valid for one narrow case. The variable nature of the actual implant conditions means that this narrow bandwidth element is at a disadvantage when operating over the entire set of possible implant environments.
U.S. Pat. No. 7,554,493 refers to a folded monopole antenna for an implantable medical device which is in particular used as RF telemetry antenna. The folded monopole antenna is electrically coupled to a metal pad located on an internal circuit board. This metal pad is coupled to a transceiver circuit for receiving and transmitting signals. Further, a circuit ground is provided which is coupled to a second metal pad and which is further coupled to a metal housing portion. The metal portion acts as antenna ground plane which effectively lengthens the antenna by lengthening the current path. The antenna is constructed of a wire or thin conductive strip that is conformable inside a biocompatible, dielectric portion of the housing. The shape of the folded antenna inside the epoxy portion includes two arcs, which arcs are connected together on one end of each arc. The antenna is thereby folded in a manner to provide the longest antenna length and maximum possible separation between the antenna and the metal portion of the housing in order to minimize interference. The folded monopole antenna described in this document is designed in such a way as to give a resonance at one particular frequency, but it is narrow in bandwidth, and thus also less appropriate for operation over the wide range of implant environments.
Higher bandwidth antennas are desirable for use in medical device telemetry because this allows for more efficient operation of the antenna over the variety of implant conditions (depth of implanted device, orientation and location of device, etc.), patient anatomies (e.g., the fat content of the tissue surrounding the implanted device can vary with age/over device lifetime, gender, etc.), and general patient-related variability (exact electromagnetic properties of the surrounding tissue). For example the dielectric constant of biological materials can vary by a factor of 10 (for example, at 403.5 MHz, the dielectric constant of fat is 5.6, while for muscle it is 57.1). This variability leads to a significant impact on the antenna performance.
In order to improve bandwidth for a telemetry antenna in an implantable medical device the article “Dual-Band Implantable Antennas For Medical Telemetry: A Fast Design Methodology And Validation For Intra-Cranial Pressure Monitoring” by A. Kiourti et al., Progress In Electromagnetics Research, Vol. 141, pages 161-183, 2013 proposes to provide one portion of the antenna that is resonant at some frequency and a second portion providing a second resonance which is slightly different but nearby.
With the same goal the document “Novel Design of Broad-Band Stacked Patch Antenna” by B. Ooi et al., IEEE Transactions on Antennas and Propagation, Vol. 50, No. 10, October 2002 suggests an E-shaped patch, also in combination with a stacked square patch.
G. Clementi et al. proposes, in the article “A Novel Low Profile Tapered Slot Antenna With Absorbing Material For Radar Imaging System”, 7th European Conference on Antennas and Propagation (EuCAP), 2013, a double exponentially tapered slot antenna for microwave imaging applications. To extend the antenna's impedance bandwidth toward low frequency, an absorbing material which is stacked on the antenna sides is provided.
For implantable medical devices it is not applicable to introduce additional layers as proposed by B. Ooi et al. above due to space limitations. The same applies for introducing a second resonance with the same element as in the method described in the article of A. Kiourti et al. above. To add some resistive/lossy material, as suggested by G. Clementi et al. above, where some of the energy is dissipated, the resonant behavior is reduced (at the expense of efficiency), which is undesirable because the extra power is burned in the lossy material and some of the energy is dissipated as heat. In addition, the used materials do not seem suitable for usage in a medical device header, for biocompatible reasons.
The present invention is directed toward overcoming one or more of the above-mentioned problems.